1. Field of the Invention
The present invention relates to methods for loading beneficial agents into implantable medical devices, and more particularly to methods for improving inkjet printing technology, specifically inkjet printing precision, with respect to loading beneficial agents into implantable medical devices.
2. Discussion of the Related Art
A common operational mode in inkjet technology, known as drop-on-demand ejection, is used as a way to deliver a controlled quantity of material to a precise location on a target. This operational mode employs the ejection of individual or the ejection of a sequence (burst) of drops based on a timed trigger event. For applications such as electronics, drug-device combinations and the like, the precision in location and amount of material delivered is critical to have a quality product since only about 1 to 10 drops are delivered at every location. Hence to accurately target the amount of material delivered, one needs to know the exact mass of every drop that is being deposited.
Since individual drops weigh in the range of 10 ng to 1 ug, it is very difficult to determine their mass accurately, even in an off-line mode. This problem is further complicated by complex geometry and machine design used for the actual deposition of drops. Hence on-line measurement of drop size and feedback control during deposition is extremely challenging. As a result, a calibration scheme is employed where a large number of drops (5000 to 20000) is collected and weighed to determine the average mass of ejected drops. This scheme assumes that the drop mass remains the same no matter how many drops are ejected. Some recent publications have shown that this assumption is not valid and the first few drops ejected in a stream of drops have a different mass than subsequent drops. Because of this discrepancy between calibration and actual deposition, the actual product may not receive the correct amount of the desired substance.
It has also been found in the course of experimentation related to the present invention that the weight of the first few drops changes as a function of the voltage amplitude used to create these drops. Hence the difference between the average mass calculated using the above calibration procedure and the average mass of the first 1 to 20 (approximately) drops changes as a function of voltage amplitude.
Accordingly, there exists a need for overcoming the disadvantages associated with the current technology by developing a methodology for depositing the exact same amount of a particular substance at various well defined locations on an object of interest.